This application is focused on the effects soy phytochemicals on preventing the progression of estrogen receptor-positive [ER(+)]and HER2-overexpressing breast cancer (BRCA). The hypothesis is that soy isoflavone genistein has a potent anti-progression effect on ER(+)/HER2-overexpressing human breast cancer in part by modulating HER2 and ER-alpha signaling pathways. The rationale for supporting this hypothesis is based on previous research and our preliminary findings, as summarized in the followings: (1). Nearly 40% of ER(+) breast tumors do not respond to antiestrogen treatment such as tamoxifen (TAM). (2) Overexpression of the HER2 gene has been shown to play a critical role in the development of hormone-insensitive BRCA from the hormone-sensitive phenotype. (3) Our preliminary studies indicate that soy isoflavone genistein significantly inhibited the growth of ER(+)/HER2-overexpressing BT-474 human BRCA cells in vitro in part via down-regulation of ERalpha and HER2. It is possible that genistein may be an effective agent for the prevention and treatment of HER2-overexpressing human breast tumors. This pilot application is to explore the possible role of genistein and genistein-rich soy phytochemicals in prevention of HER2-overexpresisng BRCA and to elucidate the possible underlying mechanisms by which genistein inhibits HER2-overexpressing breast tumors. Specific aim 1 is to determine the growth-inhibition effect of genistein and genistein-rich soy phytochemicals on BT-474 cells in a clinically relevant breast tumor model. HER2-overexpressing BT-474 breast cancer cells will be implanted into the mammary fat pad of the mice supplemented with estrogens to develop ER(+)/HER2-overexpressing breast tumor. Genistein, soy phytochemicals mixture (soy phytochemical concentrate) and TAM, alone and genistein and TAM combinations, will be evaluated for their growth inhibition activities to BT-474 tumors. Specific aim 2 is to determine the molecular mechanisms by which genistein effectively inhibits the growth of HER2-overexpressing BRCA. A series of tumor biomarkers will be determined to elucidate the possible mechanisms of action. We will determine the biomarkers that are related to estrogen-pathways such as tumor expressions of ERalpha and ERbeta. We will then quantify tumor levels of HER2 and EGFR, and MAPK and PI3K/Akt. Tumor proliferation index, apoptotic index tumor microvessel density will also be determined. These in vivo bioassays and mechanistic studies will be expected to provide significant insight into the future consideration of applying genistein and/or genistein-rich soy phytochemical fractions, alone and/or in combination with TAM, for prevention and treatment of ER(+)/HER2- overexpressing breast cancer.